Pertungstic Acid, H₂WO₅

By the action of hydrogen peroxide on solutions of tungstic acid and tungstates, compounds analogous to the persulphuric acids and persulphates are obtained. By dissolving tungsten trioxide in a 2 per cent, hydrogen peroxide solution at the boiling-point, and evaporating the resulting solution, a yellow amorphous mass results, of composition WO₃.H₂O₂.H₂O and soluble in water to an acid solution.

Several sodium salts have been prepared:

Na₂W₂O₈.2H₂O is obtained as white radiating crystals by the addition of hydrogen peroxide to a hot solution of sodium paratungstate and evaporation of the mixture in a vacuum. The crystals are easily soluble in water.

Na₂W₂O₉.6H₂O results as a yellowish-white crystalline powder when the above mixture of paratungstate and hydrogen peroxide is precipitated with alcohol. In this salt the ratio of WO₃ to active oxygen, as determined by titration with potassium permanganate in sulphuric acid solution, is 1:1, whilst in the above dihydrate it is 2:1; this suggests that the latter is derived from an acid of constitution 2WO₃.H₂O₂, i.e. H₂W₂O₈, and the former from an acid of constitution WO₃.H₂O₂, i.e. H₂WO₅, of which H₂W₂O₉ is an anhydro-acid (i.e. 2H₂WO₅ - H₂O ).

By the addition of cooled hydrogen peroxide and aqueous sodium hydroxide solutions to a cooled solution of sodium pertungstate, prepared according to the former method described above, precipitating with alcohol at -12° C. and drying the resulting product in the air, compounds of composition Na₂O₂.WO₄.H₂O₂ and Na₂O₂.WO₄. H₂O₂ + (Na₂O₂)₂WO₄ + 7H₂O have been obtained. These are unstable compounds, although more stable than the corresponding molybdenum compounds; they gradually lose oxygen on standing in air, whilst water causes rapid evolution of oxygen, hydrogen peroxide being found in the resulting solution.

The potassium salt, K₂O₄.WO₄.H₂O, is similarly produced by the action of hydrogen peroxide and potash on potassium pertungstate. It is more stable than the sodium salts, but explodes when rubbed or heated to 80° C.